1. Field of the Invention
The present invention relates to a hydrophilic resin composition, and more particularly to a hydrophilic resin composition which has good storage stability at room temperature, which is cured at a relatively low temperature in a short time and which can form a film superior in water resistance, solvent resistance, adhesivity to base material, etc.
2. Description of the Prior Art
Acrylic copolymer resins, polyurethane resins and polyester resins, which are water-soluble, water-dilutable or water-dispersible, are hydrophilic resins and accordingly free from fire hazard and pollution of working environment and, moreover, give a coating film superior in mechanical strengths, flexibility and solvent resistance. Therefore, they are in wide use as a resin component of coating, ink, surface treating agent, coating agent, adhesive, sealing agent, etc. The above resins, however, each contain a large amount of a hydrophilic group for making them water-soluble, water-dilutable or water-dispersible and have been low in water resistance.
In order to solve the above problem, there were made in recent years, attempts of adding to the above hydrophilic resin, a crosslinking agent such as aminoplast resin (e.g. water-soluble melamine resin), epoxy resin, aziridine compound or the like to allow the hydrophilic resin to be able to give a coating film of improved properties (e.g. improved water resistance). In curing the above composition of the hydrophilic resin and the crosslinking agent to obtain a cured film, however, there are disadvantages, for example, a high temperature is required for heating them and the crosslinking agent gives a problem to the working environment.
Meanwhile, there were reported production of an aliphatic polycarbodiimide resin which is cured at a relatively low temperature and which gives a working environment of high safety, and utilization of such a polycarbodiimide resin. For example, in JP-A-59-187029 and JP-B-5-27450 are disclosed an isophorone diisocyanate-derived polycarbodiimide resin and a method for crosslinking a hydrophilic resin using the polycarbodiimide resin.
The above crosslinking of a hydrophilic resin utilizes a reaction between the carbodiimide group of polycarbodiimide and the active hydrogen of active-hydrogen compound. The reaction includes, for example, one between carbodiimide group and a carboxylic acid residue present in hydrophilic acrylic resin.
Conventional aliphatic polycarbodiimides (e.g. a polycarbodiimide derived from isophorone diisocyanate), however, are highly reactive. Therefore, they give rise to a reaction even after their addition to a hydrophilic resin, providing a resin of poor storage stability.
Hydrophilic resins to which a conventional aliphatic polycarbodiimide resin is added, further have a problem in that when they are coated on an olefinic base material such as polypropylene, polyethylene or oriented polypropylene, the resulting film tends to be peeled off.
Further, conventional aliphatic polycarbodiimides, when added to a resin for aqueous coating, etc., must be emulsified beforehand together with a surfactant, mechanically by the use of an expensive high-shear mixer, making complicated the emulsification operation. Moreover, conventional aliphatic polycarbodiimides, when emulsified, inevitably generate urea owing to the reaction with water, making short the storage stability of the emulsion.